1. Field of the Invention
This invention relates generally to well production equipment, and in particular to an electro-hydraulic submersible pump.
2. Description of the Prior Art
Various types of artificial lifts are used to bring liquids to the surface of the earth when the pressure of the liquid-bearing reservoir is insufficient to produce the formation fluids by natural means. The pumping motion of the artificial lift may originate at the surface of the ground, or below, as a result of the application of electrical or fluid power to a subsurface pump. In the usual surface powered rig, a vertically reciprocating pump element at the bottom of the well is actuated by a walking beam pivotly mounted on a Sampson post and connected at one end to a sucker rod string and at the other end to a prime mover which supplies power through Pitman gear for producing the reciprocating motion of the sucker rod string.
Generally, in the surface powered rig, the prime mover consists of an internal combustion engine or electric motor. The cost of this prime mover, as well as its operation and maintenance, is, in many instances, a significant economic factor in the production of liquids from subterranean liquid-bearing reservoirs. The sucker rods are characterized by a short, fast stroke, resulting in low pump efficiency, high power consumption and low recovery rates.
An additional limitation of surface-powered rigs which operate a sucker rod through rigid production tubing is that, for practical purposes, the weight of the sucker rod for wells having a producing formation at about 10,000 feet or deeper is excessive, resulting in stretching and early failure of the rods. Moreover, the sucker rod assembly is subject to severe wear in slant-hole or crooked-hole wells. In slant-hole wells, of the type typically drilled offshore, the sucker rod is subjected to severe frictional wear because of the slant of the hole, and is therefore subject to early failure and requires frequent replacement. Frictional wear is also a serious problem in crooked holes in which the well bore follows a helical path. Because of the difficulty of drilling a vertical well at other than shallow depth, effective use of the sucker rod pumping assembly is substantially limited to shallow wells.
For the foregoing reasons, there has been considerable interest in improving pumping systems in which the motive force is provided by electrical or pneumatic/hydraulic power applied to a subsurface pump, thereby eliminating the sucker rods and affording precise control of the pumping action.
Submersible pumps of the type in which the motive force is derived from electrical, hydraulic or pneumatic power applied downhole represent a significant improvement over the reciprocating sucker rod approach. According to conventional practice, the submersible pump which is driven downhole by the electrical/hydraulic or pneumatic driving means is supported at the end of a long string of rigid production tubing joints which are coupled to each other by pin and box connections. Power conductors, signal conductors and hydraulic/pneumatic conduits are bundled together in an external umbilical cable assembly secured to the rigid production tubing joints, and are coupled to the downhole pump.
Conventional hydraulically operated submersible pumps commonly employ a reciprocating piston within a stationery cylinder, with a slender connecting rod being used to connect the hydraulic motor to the production pump. Hydraulic fluid is conveyed under pressure through long conduit lines which extend from a surface pumping facility through an umbilical conduit line to the drive piston. Although this arrangement has achieved a measure of success, its usefulness is limited to relatively shallow wells, because of the relatively high pressure drop across the long hydraulic conduits. A further limitation on the hydraulic pump assembly is imposed by the slender rod which is commonly used to connect the drive piston to the production piston within the pump. Since the slenderness ratio of the connecting rod is unfavorable for action in compression, the effective length of the pump stroke must be maintained at a low value and the speed of operation maintained at a moderately high value in order that a sufficient quantity of formation fluid can be pumped. This results in a churning action which tends to cause the formation of emulsion.